Gel electrophoresis is a well established technique for the analysis of protein and nucleic acid preparations. In the most common way to carry out electrophoresis, a thin gel slab is cast in a mold, samples are applied at one end of the gel and an electric voltage is applied over the gel to make the sample molecules migrate electrophoretically in the field. The migration rate depends on the charge-to-mass ratios of the molecules as well as on their sizes, due to the sieving action of the gel medium. To improve the separation it is common to use gradient gels, where the solids content of the gel increases continuously in the migration direction. It is also common to introduce other spatial patterns in the gels, such as a sample application/stacking zone with lower solids content at the sample application end of the gel. In an alternative gel electrophoresis technique, isoelectric focusing (IEF), a pH gradient is formed over the gel and the sample molecules migrate electrophoretically to the point in the gradient where pH is equal to the isoelectric point of each molecule.
For high resolution gel electrophoresis, crosslinked polyacrylamide is the most common gel material. A casting solution of acrylamide (monomer), N,N′-methylene bisacrylamide (crosslinker), charged acrylamides (for IEF), buffer components, initiator components and optionally other components is poured or pumped into a mold and polymerization is initiated to convert the solution to a solid gel slab. A number of other monomer/crosslinker combinations can also be applied in a similar way. Monomer solutions of low viscosity are preferred to ensure easy and complete filling of narrow molds.
To create gradients, two casting solutions of different composition (typically differing in monomer concentration and/or monomer type) are mixed together by pumps or a special gradient mixer and the gradient solution is continuously pumped at low flow rate into one end of the mold. The mold is then filled with a casting solution having a gradient in the composition and, after polymerisation, a gradient gel slab will be formed. When a step change in gel composition is desired, such as for a sample application/stacking zone having lower concentration than the separation gel, it is more common to fill the mould with one casting solution (or a gradient), polymerize, fill the remaining mold space with another casting solution and then polymerise again. Both the gradient pumping and the double polymerization method are time-consuming and complicated to carry out and there is a need for more efficient methods.
JP2004007393 describes a non-contact ink-jet printer intended for application of agarose or polyacrylamide gels in patterns for electrophoresis. No details are however given on the formulation of the casting solutions. If standard polyacrylamide gel casting solutions are used with patterning techniques like this, no useful gels will be produced.